The present invention relates to multi-ratio gearbox housing for a vehicle, wherein said housing being made of light metal material. The housing comprises a first bearing seat having a cylindrical inner surface, a reinforcement member having a first cylindrical inner surface surrounding the first bearing seat for reducing thermal radial expansion of the first bearing seat. The reinforcement member being of a material the coefficient of thermal expansion of which is less than the coefficient of thermal expansion of said light metal material of said housing.
Fix-axes gear transmissions are used in many fields of technology, for instance in vehicles in manually shifted or automated form. They comprise a number of gearwheels that are located on a number of shafts. The shafts are supported by bearings that are seated in bearing seats of the transmission housing. The rotational axes of the shafts are thereby substantially fixed, hence the designation fix-axes gear transmissions. Modern heavy duty truck engines with increasing power output levels result in increased heat generation within the gearbox due to friction losses. It is desirable to push the upper temperature operation limit of the gearbox further to allow a broader operation range, facilitating handling of increased power output without further expensive and complex cooling arrangements. However, when increasing the allowable gearbox operation temperature, increasing problems with rotating outer bearing rings are indentified. This problem occurs mainly due to the different level of thermal expansion of the housing and the outer bearing ring. The housing is normally made of a light metal material having a first coefficient of thermal expansion, and the bearing outer ring is normally press-fitted within the bearing seat and made of steel having a second coefficient of thermal expansion, which is lower than said first coefficient of thermal expansion. The diameter of the bearing seat in said housing thus tends to expand more than the diameter of the outer bearing ring, such that the press-fit of the outer bearing ring decreases, and even completely disappears. As a result, the outer bearing ring may rotate within its bearing seat, leading to abnormal wear of the housing and increased vibrations and friction losses.
One known solution for preventing rotation of outer bearing rings is to secure them to the gearbox housing by means of axial pins that are inserted after mounting of the bearings. These pins thus mechanically lock the outer bearing ring with the housing. This solution however requires additional components, additional assembly steps, and reduced reliability as the pins may break or get loose within the gearbox.
In another solution, as shown in U.S. Pat. No. 4,809,833, a cylindrical clamping ring is seated on cylindrical outer surface of the bearing housing, for the purpose of limiting the widening of the bearing seat upon increased working temperature. This solution is however limited to certain specific housing designs having said cylindrical outer surface of the bearing housing.
There is thus a need for an improved multi-ratio gearbox housing, which at least partly removes the above mentioned disadvantages.
It is desirable to provide an improved multi-ratio gearbox housing where the previously mentioned problems are partly avoided. According to an aspect of the present invention, a reinforcing member being a cast-in insert arranged in said housing, and in that the radial distance between said first cylindrical inner surface of said reinforcement member and said cylindrical inner surface of said first bearing seat is substantially constant along circumferential lines of said first bearing seat, thereby facilitating improved form stability of said cylindrical inner surface of said first bearing seat over the temperature operating range of the housing.
According to another aspect of the invention, a method of manufacturing a multi-ratio gearbox housing is provided, which comprises the steps of providing a casting mould for said housing that comprises a first bearing seat having a cylindrical inner surface, arranging at least one reinforcement member within said mould, such that a first cylindrical inner surface of said reinforcement member surrounds said first bearing seat for reducing thermal radial expansion of said first bearing seat in the finished product, casting light metal material into said mould, wherein said reinforcement member being of a material the coefficient of thermal expansion of which is less than the coefficient of thermal expansion of said light metal material of said housing, and wherein the radial distance between said first cylindrical inner surface of said reinforcement member and said cylindrical inner surface of said first bearing seat is constant along circumferential lines of said first bearing seat, facilitating improved form stability of said cylindrical inner surface of said first bearing seat over the temperature operating range of the housing.
The inventive solution is based on the fact that the reinforcement member does not expand as much as the light metal material of the housing due to the difference in coefficients of thermal expansion, thereby holding, and preventing the light metal material of the bearing seat from expanding outwards. Instead, the light metal material inside of the reinforcement member may even expand inwards, thereby maintaining bearing seat diameter intact over a wide thermal operating range of the gearbox housing, thus preventing the bearing outer ring from rotating.
Further advantages are achieved by implementing one or several of the features of the dependent claims.